The present invention relates to a zoom lens system, and more particularly to a miniature zoom lens system comprising only five lens elements. The zoom lens system of the present invention is particularly suited for use with lens shutter cameras and provide the several advantages of short total length and small outside diameter.
Zoom lens systems have been widely used in cameras nowadays because it is capable of conveniently zooming the focal length of an easy lens system to facilitate the use of the camera. Generally, the zoom lens systems fall into two types. The first type belongs retro-focus lens system, which typically comprises a front lens group having a negative refractive power and a rear lens group having a positive refractive power. An airspace is reserved between the front lens group and the rear lens group, and it is possible to effectuate variation of focal length of the lens system by varying the airspace. Such a zoom lens system has the advantage in that it allows an ample space to be reserved for arranging a mirror. Consequently, the first type of zoom lens system is suitable for use in the well-known single lens reflex camera. However, it has disadvantages in that the front lens group of such a lens system has a large diameter and long total length. These disadvantages make it impossible to make the zoom lens system compact.
The second type of zoom lens system is the so-called telephoto system, which comprises a front lens group having a positive refractive power and a rear lens group having a negative refractive power. An airspace is similarly provided between the front lens group and the rear lens group for zooming. This type of zoom lens system is characterized in that it has a shorter back focal length and therefore allows the total length of the zoom lens system to be shortened, so that it is especially suitable for use in lens shutter cameras.
In the prior art, it is known that the total length of the zoom lens systems may be shortened by reducing the radii of the curvature of the lens elements. However, it will cause large aberrations during zooming. In the alternative, in order to limit the radii of the curvature of the lens elements and maintain the quality of lens, it may be necessary to employ more lens elements to form the lens systems. However, employing more lens elements it will increase the cost and the resultant cameras cannot be desirably light weight.
To overcome the problems mentioned above, various prior art compact zoom lens systems have been developed in this field. For example, U.S. Pat. No. 4,830,476 discloses a compact zoom lens system comprising four lens elements in the lens unit. The front lens group of the zoom lens system comprises a negative lens element, and a positive lens element and the rear lens group comprises a positive lens element and a negative lens element.
U.S. Pat. No. 4,838,669 discloses a zoom lens system with a front lens group having positive refractive power and a rear lens group having negative refractive power. The front lens group of the lens system comprises a positive meniscus lens, a negative lens and at least one positive lens, and the rear lens group of which consists of a positive lens, a biconcave lens and a negative meniscus lens.
U.S. Pat. No. 4,682,860 discloses a zoom lens system comprising a front lens groups having a positive refractive power and a rear lens group having a negative refractive power. The first lens group includes at least two positive lens elements and at least one negative lens element; the second lens group includes at least one positive lens element and at least one negative lens element. It is noted that the fourth embodiment as shown in FIGS. 9A and 9B of the '860 patent comprises five lens elements forming a zoom lens system. However, the lens system disclosed in the '860 patent contains lens elements with relatively steep radii of curvature and thus is relatively costly to manufacture.
U.S. Pat. No. 4,991,945 discloses a zoom lens comprising five lens elements. The first three elements form a positive lens group and the last two elements form a negative lens group. All of the lens elements in this prior patent are spherical and the optical condition/parameters such as radii of curvature of lens elements are different from the present invention.